Today, Czochralski-grown (CZ) silicon substrates are the most commonly used substrates in making semiconductor devices. The CZ-grown silicon substrates typically contain 25-35 parts per million atomic (ppma) of oxygen, virtually all of which is interstitial (between silicon lattice sites). The oxygen can form precipitates and generate crystal defects in the silicon substrates during thermal processing steps. The precipitates and related crystal defects within the device active region may cause device degradation and need to be reduced or eliminated within the active region of the devices.
Typically, the oxygen concentration in the active region of the substrate is reduced by the performing a furnace cycle to form a denuded zone. During the heating, the oxygen near the surface of the silicon substrates diffuses out of the substrates. The step is usually performed with an inert gas with or without an oxygen bleed. The use of an oxygen bleed actually inhibits denuding (that is a type of diffusion process) relative to using an inert gas without an oxygen bleed. However, using only an inert gas during the denuding also has problems. An inert gas typically contains more than one part per million of oxygen. Even at this concentration, denuding is still inhibited. Also, substrate pitting may occur. The pitting is believe to be caused by the substrate reacting with oxygen to form a volatile silicon-oxygen compound.
An attempt to denude the substrate without inhibiting denuding or causing pitting is to use hydrogen during the denuding process. Unfortunately, hydrogen is flammable. The lower flammability limit of hydrogen in air is about 4.65 percent. Also, hydrogen is more difficult to seal within a chamber compared to relatively heavier gases, such as nitrogen or oxygen. In order to overcome the problems, a special and expensive furnace is used for the hydrogen-based denuding process. As a result, the cost of the hydrogen-based denuding process is high.